Hepatic steatosis assessment using ultrasound homodyned-K parametric imaging: The effects of estimators

Background: The homodyned-K (HK) distribution is an important statistical model for describing ultrasound backscatter envelope statistics. HK parametric imaging has shown potential for characterizing hepatic steatosis. However, the feasibility of HK parametric imaging in assessing human hepatic steatosis in vivo remains unclear. Methods: In this paper, ultrasound HK μ parametric imaging was proposed for assessing human hepatic steatosis in vivo. Two recent estimators for the HK model, RSK (the level-curve method that uses the signalto- noise ratio (SNR), skewness, and kurtosis based on the fractional moments of the envelope) and XU (the estimation method based on the first moment of the intensity and two log-moments, namely X- and U-statistics), were investigated. Liver donors (n=72) and patients (n=204) were recruited to evaluate hepatic fat fractions (HFFs) using magnetic resonance spectroscopy and to evaluate the stages of fatty liver disease (normal, mild, moderate, and severe) using liver biopsy with histopathology. Livers were scanned using a 3-MHz ultrasound to construct μ_RSK and μ_XU images to correlate with HFF analyses and fatty liver stages. The μ_RSK and μ_XU parametric images were constructed using the sliding window technique with the window side length (WSL) =1.9 pulse lengths (PLs). The diagnostic values of the μ_RSK and μ_XU parametric imaging methods were evaluated using receiver operating characteristic (ROC) curves. Results: For the 72 participants in Group A, the μ_RSK parametric imaging with WSL =2.9 PLs exhibited similar correlation with log10(HFF), and the μ_RSK parametric imaging with WSL = 3 PLs had the highest correlation with log10(HFF) (r=0.592); the μ_XU parametric imaging with WSL =1.9 PLs exhibited similar correlation with log10(HFF), and the μ_XU parametric imaging with WSL =1 PL had the highest correlation with log10(HFF) (r=0.628). For the 204 patients in Group B, the areas under the ROC (AUROCs) obtained using μ_RSK for fatty stages . mild (AUROC1), . moderate (AUROC2), and . severe (AUROC3) were (AUROC1, AUROC2, AUROC3) = (0.56, 0.57, 0.53), (0.68, 0.72, 0.75), (0.73, 0.78, 0.80), (0.74, 0.77, 0.79), (0.74, 0.78, 0.79), (0.75, 0.80, 0.82), (0.74, 0.77, 0.83), (0.74, 0.78, 0.84) and (0.73, 0.76, 0.83) for WSL =1, 2, 3, 4, 5, 6, 7, 8 and 9 PLs, respectively. The AUROCs obtained using μ_XU for fatty stages . mild, . moderate, and . severe were (AUROC1, AUROC2, AUROC3) = (0.75, 0.83, 0.81), (0.74, 0.80, 0.80), (0.76, 0.82, 0.82), (0.74, 0.80, 0.84), (0.76, 0.80, 0.83), (0.75, 0.80, 0.84), (0.75, 0.79, 0.85), (0.75, 0.80, 0.85) and (0.73, 0.77, 0.83) for WSL = 1, 2, 3, 4, 5, 6, 7, 8 and 9 PLs, respectively. Conclusions: Both the μ_RSK and μ_XU parametric images are feasible for evaluating human hepatic steatosis. The WSL exhibits little impact on the diagnosing performance of the μRSK and μXU parametric imaging. The μXU parametric imaging provided improved performance compared to the μRSK parametric imaging in characterizing human hepatic steatosis in vivo.